1. Field of the Invention
This invention relates to an exposure amount adjusting device such as a shutter or a diaphragm device to be used for a still camera, a video camera or an interchangeable lens for a camera.
2. Description of the Related Art
An exposure amount adjusting device has been disclosed in U.S. patent applications, Ser. No. 035,453 (filed Apr. 7, 1987) and Ser. No. 122,764 (filed Nov. 19, 1987) both of which were assigned to the assignees of the present application. The exposure amount adjusting device is arranged to open and close light shielding blades with a stepping motor which serves as a drive source by rotating a rotor while having a stator in a stationary state. The stepping motor is disposed away from the position of a passage provided for a light beam.
An exposure amount adjusting device of this kind comprises a diaphragm part and a driving part. The diaphragm part includes a cam, diaphragm blades for light shielding, a rotary ring, a rotary ring retaining plate, screws, etc. The driving part includes a stator, a stator guide, an exciting coil, a magnetic rotor, a rotor gear, a base plate, a driving base plate, screws, etc.
However, these numerous components of the diaphragm part and the driving part necessitate complex assembly work which is to be performed for mounting, connecting and screwing from many directions. This has caused an increase in the number of assembly and adjustment processes and thus resulted in an increase in cost. Further, the conventional exposure amount adjusting device has been arranged to have one of the bearings of the rotor of the stepping motor disposed on the driving base plate and to have the driving base plate coupled to a cam with screws. The driving base plate has been thus arranged to serve as one of the bearings of the stepping motor. This further increases the number of components. Besides, the arrangement to couple the driving base plate to the cam makes it necessary to accurately adjust a distance between the axes of the driving gears. That arrangement further increases the number of processes for assembly and adjustment work and thus has further increased the cost of the device.
Further, in the conventional exposure amount adjusting device of this kind, the rotor of the stepping motor has been prepared as follows: A stainless shaft of it is insert-molded in a plastic magnet and, after magnetization, a pinion gear is pressure-fitted on the shaft while carrying out positioning work. However, this also has caused a further increase in the number of assembly processes as well as in the number of component parts. A further drawback of the conventional device has resided in difficulty in the position adjustment of the magnetic pole of the rotor to the pinion gear. Because of this difficulty, the pinion gear is positioned by pressure fitting at random relative to the magnetic pole of the rotor. This has necessitated accurate adjustment of the aperture of the diaphragm and thus caused a further increase in cost.
Further, in the conventional exposure amount adjusting device, the stator of the stepping motor and an iron core which serves as the magnetic path thereof are made of discrete members. Hence, it has been necessary to couple the iron core to the stator by means of a screw, which also has caused an increase in the number of parts, complexity of assembly work and a still further increase in cost.
The conventional exposure amount adjusting device is provided with an opening switch for detecting the open position of the diaphragm blades. This switch is mounted on a base, which is secured to the retainer plate of the rotary ring by heat caulking. However, since the base is secured by heat caulking with a supersonic wave or the like, the base tends to have a rattling play. The play then necessitates readjustment to prevent unstable switching position of the switch. This also has increased the number of parts and the cost of the device.
Further, there ha been provided no special means for facilitating the assembly work on the cam, the rotary ring and the diaphragm blades. Therefore, in mounting the rotary ring after the diaphragm blades has been mounted on the cam, the blades tend to move to make the rotary ring mounting work difficult.
Further, in cases where the exposure amount adjusting device of this kind is arranged to have the cam and the rotary ring coupled with each other by a bayonet method in a manner, for example, as disclosed in U.S. Pat. No. 4,152,064, a stopper is discretely arranged to prevent excessive rotation of the rotary ring. In this case, however, since the stopper is a discrete member, it not only increases the number of parts but also necessitates additional work such as screwing, positioning, etc. and thus also has resulted in a further increase in cost.